1. Field of the Invention
This invention relates to transmission line fault locating systems and in particular to a method and an apparatus for determining on which side of a maintenance termination unit the fault is located.
2. Description of the Prior Art
It is becoming more and more important to find improved apparatus and methods for locating faults in a transmission line. This is especially important in the context of the telephone industry. In telephone transmission loops, many customers provide their own terminal equipment and install their own wiring.
Typically a problem arises in determining whether there is a service interrupting fault on the customer side of a network customer interface (NCI) or whether the fault has occurred on the network side of the NCI. Note that the NCI refers simply to a junction point between the telephone company and the customer. Typically, fault location is accomplished utilizing a device called a maintenance termination unit (MTU) in conjunction with signals from a remote test center.
It is also important to be able to determine whether there is actually an MTU on the transmission line in those environments where MTUs are only partially deployed. This capability makes it possible to distinguish the loops where fault location information can be obtained from those where it cannot. This capability is particularly desirable on loops where cable pair changes make MTU record keeping difficult. Previous maintenance termination units have several shortcomings: (1) they give an inaccurate count of the number of ringers connected to the transmission loop; (2) they cannot be distinguished from a tone ringer; (3) in the absence of record keeping, a loop with an MTU connected to it cannot be distinguished from an unequipped loop; (4) the termination cannot be used to verify the operation of the MTU, and finally (5) a power fault of sufficient magnitude at the MTU will cause the termination to open circuit. These deficiencies individually do not cause a severe problem but collectively they can present various problems in determining fault location.
What is needed, is a maintenance termination unit that overcomes the above-mentioned problems and also is a simple and reliable apparatus. In addition, the unit must be easily adaptable to transmission loops in general and, more particularly, to telephone transmission loops to readily ascertain the location of a service interrupting fault.